A fibre Bragg grating is contained within the core of a short segment of optical fibre that reflects particular wavelength of light and transmits all others. The fundamental principle behind the operation of a fibre Bragg grating is Fresnel reflection. Light traveling between media of different refractive indices may both reflect and refract at the interface. This is achieved by successive perturbations to the effective refractive index in the core of the optical fibre. These perturbations to the effective refractive index lead to a reflection of a portion of the light which propagates along the fibre. Any changes in refractive index, as a result of strain within the fibre due to change in temperature, pressure and the like, will cause proportional shifts in the reflected spectra. A fibre Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.
The Bragg wavelength is defined by Eq. 1λB=2neffΛ  Eq. 1where Λ is the grating period, neff is the effective refractive index of the grating in the core of the fibre and λB is the Bragg wavelength or the reflected wavelength. From this relationship, any changes to the grating period or the effective refractive index of the grating in the core of the fibre will cause a change in the Bragg wavelength.
In-fibre Bragg gratings are used extensively as sensors for various parameters including displacement, strain, pressure, humidity, radiation dose and temperature. Temperature-sensing using fibre Bragg grating works on the principles of thermal expansion and photo-thermal effect. Photo-thermal effect is the changing of the refractive index due to change in temperature, and thermal expansion is changing the grating period due to change in temperature. Pressure measurement is important in environmental applications, medical diagnostic, research and development, and many fields of engineering. Specifically, fibre Bragg gratings are useful in instrumentation applications such as seismology and down-hole sensors in oil and gas wells to measure the effects of external pressure, temperature, seismic vibrations and inline flow. As such, they offer a significant advantage over traditional electronic gauges used for these applications since they are less sensitive to vibration or heat, and consequently they are far more reliable.
Fibre Bragg gratings are an attractive alternative to other piezoelectric, resistive or other solid-state sensing technologies because they are small (typically 125 μm in diameter), mechanically compliant, intrinsically robust, chemically inert, resistant to corrosive environments, immune to electromagnetic interference, and are capable of simultaneous multi-parameter sensing when suitably configured. Moreover, fibre Bragg grating sensors can be multiplexed along a single optical fibre, thereby allowing spatially distributed measurements.
Fibre Bragg gratings are applied only on a limited basis in medical pressure measurement applications, primarily because bare fibre Bragg gratings possess low sensitivities to hydrostatic pressure and are only capable of resolving pressure variations of the order MPa. In an effort to increase the sensitivity of fibre Bragg gratings to pressure, mechanical amplification schemes such as polymer coatings around the fibre circumference, pressure spacers and diaphragms, and chemical etching that reduces the cross-sectional area of the optical fibre have been developed. Sensors utilizing these schemes have increased pressure sensitivity because the strain along the fibre Bragg gratings is concentrated relative to the case of a bare-fibre Bragg grating. Prior art has presented a glass-bubble (4 mm diameter) housed fibre Bragg grating sensor and a polymer coated fibre Bragg grating sensor all with major diameters of the millimeter order. Due to their increased size, these sensors are invasive for medical pressure measurement applications, and they do not retain the intrinsic benefits offered by fibre Bragg gratings.
The prior art discloses a medical sensor which has a bio-compatible fibre optic sensor probe for invasive medical use that includes an optical fibre, a sensing location at which the fibre is configured to provide at least one detectable changeable optical property responsive to a strain within the fibre, and at least one sensing element which undergoes a volumetric change in response to an in-body parameter to be sensed. The sensing element is coupled to the fibre in such a way that the volumetric change induces strain within the fibre so as to vary the detectable changeable optical property. In one embodiment, a fibre Bragg grating is used and a thick polymer coating such as a compliant silicone is used as the sensing element. The polymer coating on the fibre increases the strain on the fibre when exposed to pressure on its outer cylindrical surface. The prior art also discloses a fibre Bragg grating-based intervertebral disc pressure sensor that has both amplified sensitivity to pressure (i.e. seven times that of a bare fibre) as well as a diameter of 400 μm and a sensing area of only 0.126 mm2.